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YD Floating Crane

A Floating crane is equipment designed by the manufacturer (or employer) for marine use by permanent attachment to a barge, pontoons, vessel or other means of flotation. Floating cranes are also known as crane ships [if they are self-propelled] or floating cranes [if they are not]. Floating cranes are cranes whose floating base, i.e. a pontoon, mounts a crane slewing portion, i.e. a framework adapted to rotate relative to a slewing ring secured on the pontoon. Installed on this framework is a boom with boom luffing, boom slewing and load hoisting drives controlled by respective master controllers. The pontoon accommodates a heel compensation means, comprising a counterweight arranged on a trolley installed on rails laid on the pontoon. The master controllers of the boom slewing, boom luffing and load hoisting drives are switched on and off manually by the crane operator according to visually observed readings of the angle-of-heel transmitter. Such a control of said drives requires constant attention on the part of the crane operator which reduces the operating reliability of the crane.

Floating cranes can be divided into self-propelled and non-self-propelled according to the hull maneuverability. The self-propelled type relies on the ship's own propulsion device and has the technical performance of forward and turning. Non-self-propelled floating cranes still need to be able to make short-distance shifts during loading and unloading work to achieve changes in spreader operating positions and complete range adjustments.

Floating cranes have been provided for building bridges, loading and unloading from docks, etc. which have typically included a rectangular barge structure upon which a crane is carried. Such rectangular barges typically range up to 75 meters in length and 20 meters width, accommodating booms up to 100 meters in length. When the crane boom carried on the barge swings laterally, the end of the barge loaded by the boom dips into the water placing side loads on the boom which, under certain severe conditions, can cause collapse of the boom. The resulting tilting of the barge to one side as the boom swings, makes accurate positioning of the boom difficult and makes the whole structure out of level. Waves from natural causes or from passing ships or even a small yacht can cause considerable lift and side loading of the boom structure.

Furthermore, the rectangular floating crane structure cannot be closely positioned to an obstacle during use since it is awkward and cannot be maneuvered in close without a dangerous likelihood of striking the obstacle. Boom lengths have been limited on rectangular barges since the problems of maintaining the floating apparatus level and boom side loading increases with the length of the boom.

Cranes, such as of the boom type, have long been mounted on barges, floating platforms, or other boats and utilized for various purposes, such as loading and unloading of ships and barges, ship repair, reclaiming sunken ships, and bridge building. The size of crane barges must be limited, especially the width, to permit passage through locks, canals and other narrow inland waterways. Similarly, other economic or design constraints may limit the length, breadth or draft of any type of floating vessel.

In use of crane barges, extremely heavy loads, several times the unloaded displacement of the barge, sometimes need to be so lifted to considerable horizontal distances from the barge, particularly for bridge building. When utilizing a barge-mounted crane for lifting these heavy loads, such as a large section of a bridge weighing hundreds of tons, the barge may list or trim severely, and the draft will increase substantially. If the inclination, especially listing, should become too great, the crane barge may become unstable, and could capsize rather than right itself. At lesser angles the deck of the barge may be an unsafe working surface. Such tilting of the barge may make control of the crane uncertain. Likewise, any type of boat or vessel which normally has positive stability, conventionally defined by having its center of gravity beneath its metacentre, may become unstable when a large external moment is applied, such as caused by improper loading or wave action.

The truss-like boom of a barge-mounted crane, for example, which may be several times longer than the length of the barge, has little strength except in its plane. Since the crane is normally positioned aft of the center of flotation of the barge to facilitate lifting over the end of the barge, lifting over the side of the barge may cause the crane boom to tilt aft from its normal substantially vertical plane due to aft sinkage of the barge. Similarly, lifting over a corner of the crane barge, rather than directly over an end or side, may cause a similar condition due to simultaneous listing and trimming of the barge. Also, the crane boom is subject to high wind loads normal to the vertical plane of the boom, which effects sideward shear on the boom and a moment at the connection of the boom to the barge, causing the barge to list or become out of trim and the plane of the boom to tilt from vertical. In these cases when the boom tilts from vertical, any suspended load presents a sideward component of force which is likely to collapse the boom. Tilting out of the vertical by one degree or even less may endanger the crane. Other types of vessels may be endangered by unsymmetrical loads, whether or not accompanied by wind forces.

In conventional floating cranes with a derricking jib it is the practice for the purpose of increasing outreach and luffing height to fit a supplementary top mounting jib. Top mounting jibs are mounted on laterally projecting hinge pins at the top or head of the main jib. In the case of hitherto conventional top mounting jibs which are about 20 meters in length the crane is capable of picking up the supplementary jib from the horizontal without assistance. If the top mounting jib is of greater length, external assistance is needed to lift the additional weight. For the assembly of special constructions, such as offshore rigs, great luffing heights and load lifting capacities are required. These requirements cannot be satisfactorily met by conventional supplementary jibs mounted at the top of the main jib. The lengths of rope on the derrick winch needed for erecting such systems would far exceed the rope lengths needed for luffing the loads. Moreover, the design weight of the main jib for taking up the greater loads would also have to be higher.




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Page last modified: 25-12-2019 18:43:25 ZULU